Ready-mixed concrete manufacturers, in hot weather regions are faced with drops in compressive strengths of concrete produced in summer, as shown in FIG. 1. High ambient temperatures increase the rate of evaporation from fresh concrete resulting in lower effective water content and hence lower effective water-cement ratio per weight. Reduction of compressive strength has also been observed on specimens produced under a controlled environment and tested in a laboratory. High temperature accelerates cement hydration and the bonding between the cement grains becomes weaker. Therefore, the early-age strength increases with higher curing temperatures because the reaction rate is faster, but 28-day strength decreases because of the poor bonding between cement grains at these elevated temperatures. It is noted that higher temperature aggregates results in greater concentration of calcium hydroxide at the interface. This observation leads to the assumption that the transition zone might be weakened by chemical phenomenon due to the rise of the constituent temperatures.
The weather conditions of many regions of the world are associated with hot weather for six to eight months per year. Northern and southern Africa, Arab peninsula, Southern Asia, Southern part of North America, Middle regions of Latin America, and northern Australia are the hottest regions in the world. The weather fluctuates between summer temperatures that approach 50° C. and winter temperatures that sink to 18° C. Relative humidity follows a similar pattern ranging between 5% and 90% from inland to coastal regions, respectively. Ready-mixed concrete manufacturers have to accommodate these extreme highs and lows in climate fluctuation.
Aggregate temperature plays a very important role in defining the concrete mix temperature. It has been shown that keeping the aggregate temperature about 10° C.-15° C. is adequate in achieving proper results. Cooling the concrete aggregate is one of the most effective methods to reduce the concrete mix temperature. Different methods have been developed for that purpose. In these methods cooling is obtained using; chilled-water, ice, chilled-air, or liquid Nitrogen. The key factor in choosing the proper method is the most economic technique without degrading the cement properties. The optimization analysis results will depend significantly on the amount of aggregates to be cooled down and the required temperature drop.
Cooling using liquid Nitrogen has many virtues, however, there has not been a great deal of testing. There is a lot more industry needs to know about how liquid nitrogen affects cement hydration products, concrete set, and concrete production equipment as well. Furthermore, there are safety issues that need to be addressed more fully. Chilled water is commonly used in reducing the aggregates temperature in hot weather. Cooling 600 m3 of aggregates at 7° C. requires about 300 m3 of chilled water storage which costs about $20,000. Ready mix concrete industries in hot regions, however, may require a temperature drop of about 35° C. for much bigger amount of aggregates. In such case although cooling by chilled water is an effective method it will be highly costly and not feasible to achieve such target. Furthermore, the concrete aggregates are required to be mostly dry before mixing to achieve lipophilic (oil-loving) surface for good bonding between the cement and the aggregate during mixing. Flake ice could be added to the mixing drum as a direct substitute for batched water on a pound-for-pound basis. It is reported that an ice making plant that delivers 10 tons of ice per day would cost about $300,000. Thus, medium production rate plants that require around 100 tons/hr of concrete will require a huge investment to accommodate for a proper ice maker plant. Chilled air is a preferred candidate, although this requires huge flow rates and extensive cooling systems. It is explicitly reported that chilled-air cooling is an economical option when large volumes of aggregates must be cooled with significant large temperature difference.
The above mentioned cooling methods are utilized through different cooling equipment. Among those are belt conveyors, cooling drums, chilled storage rooms, and a mix of those methods. Most of this equipment is meant for cooling and drying the foundry sand. It could be adopted, however, for sand and coarse aggregates cooling. The cooling drums are designed on the basis of well mixing the aggregates using radial webs. The cooling process in the existing drums in the market is based on mixing the aggregates to enlarge the contact area between the aggregates and the cooling air. Research shows that drying drums are very compact, efficient, and provide a high production rate. Cooling drums entail, the disadvantages of being quite expensive, consume high mechanical power, and are difficult to maintain. On the other hand, cooling using air jets over belt conveyors may be the cheapest and simplest method. However, the concrete industry reports long cooling time, low cooling efficiency, large occupied space and a low production rate. A combination of both methods is generally recommended. Three-stage cooling of return sand is effective and efficient when flash cooling and premixing are accomplished on a belt conveyor and final cooling is performed in a rotary sand blending, cooling, screening drum.
Convection to the cooling air is the main heat transfer theme in these previous designs. Cooling through the thermal contact between the aggregates and the cold belt conveyor or drum body as well as the mixing process was neither analyzed nor optimized. The heat flow during the cooling process, either by belt conveyors or drums, needs to be analyzed and optimized to achieve short and optimum cooling time with low cooling power. The main objectives of the current work are to propose optimized designs for belt conveyors system and drum cooling system to be used in cooling the concrete coarse and fine aggregates and to present a numerical simulation for the cooling process using the finite element method with the objective of optimizing the overall system performance.
Needs exist for improved systems and methods for cooling aggregates.